The present invention generally relates to an absorbent structure that forms a superabsorbent composition in situ having the ability to absorb a large quantity of liquid. The structure includes an absorbent material and a fibrous material having an activating agent incorporated therein. The fibrous material releases the activating agent in response to contact with liquid and the activating agent activates the absorbent material so that it becomes a superabsorbent material. The absorbent structure is useful in disposable absorbent products, such as those disposable absorbent products that are used to absorb bodily fluids.
The use of water-swellable, generally water-insoluble absorbent materials, commonly known as superabsorbents, in disposable absorbent personal care products is known. Such absorbent materials are generally employed in absorbent products such as diapers, training pants, adult incontinence products, and feminine care products in order to increase the absorbent capacity of such products while reducing their overall bulk. Such absorbent materials are generally present in absorbent products in a fibrous matrix, such as a matrix of wood pulp fluff. A matrix of wood pulp fluff generally has an absorbent capacity of about 6 grams of liquid per gram of fluff. The superabsorbent materials generally have an absorbent capacity of at least about 10, desirably of about 20, and often of up to about 1000 times their weight in water. Clearly, incorporation of such absorbent materials in personal care products can reduce the overall bulk while increasing the absorbent capacity of such products.
A wide variety of materials have been described for use as absorbent materials in personal care products. Such materials include natural-based materials such as agar, pectin, gums, carboxyalkyl starch, and carboxyalkyl cellulose, as well as synthetic materials such as polyacrylates, polyacrylamides, and hydrolyzed polyacrylonitrile. While such natural-based absorbent materials are known for use in personal care products, they have not gained wide usage in such products, at least in part because their absorbent properties are generally inferior compared to the synthetic absorbent materials, such as the sodium polyacrylates. Specifically, many of the natural-based materials tend to form soft, gelatinous masses when swollen with a liquid. When employed in absorbent products, the presence of such soft gelatinous masses tends to prevent the transport of liquid within the fibrous matrix in which the absorbent materials are incorporated. This phenomenon is known as gel blocking. Once gel blocking occurs, the product cannot efficiently absorb subsequent insults of liquid, and the product tends to leak. Further, many of the natural-based materials exhibit poor absorption properties, particularly when subjected to external pressures.
In contrast, synthetic absorbent materials are often capable of absorbing large quantities of liquid while maintaining a generally stiff, non-mucilaginous character. Accordingly, synthetic absorbent materials can be incorporated in absorbent products while minimizing the likelihood of gel blocking.
One property of currently available superabsorbent materials is that such materials typically very rapidly absorb the liquid that comes into contact with the superabsorbent material. While such quick absorbency of the liquid may be desirable in many applications, there are certain applications in which it is not desirable. For example, in an absorbent structure that is insulted with a liquid at only a very localized location, it would generally be desirable to have the liquid distributed throughout the entire volume of the absorbent structure so that the absorbent capacity of the entire absorbent structure is utilized. However, if the superabsorbent material located near the localized insult location absorbs the liquid very quickly, this superabsorbent material may swell and restrict the flow of the liquid throughout the rest of the absorbent structure, possibly resulting in the liquid leaking out of the absorbent structure in the area of the localized insult location. Accordingly, it is often desirable to employ a superabsorbent material in the localized insult location that actually absorbs the liquid at a slow rate. This allows the liquid to be distributed throughout the absorbent structure first and then be subsequently absorbed by the superabsorbent material.
Several methods are known to slow down the liquid absorbing rate of a relatively fast absorbing superabsorbent material. For example, it is possible to coat the fast absorbing superabsorbent material with a material that is nonabsorbent and/or hydrophobic. Such coating materials tend to temporarily shield the underlying superabsorbent material from any liquid and, thus, delay the absorption of the liquid by the superabsorbent material. However, such coating materials often reduce the overall liquid capacity of the superabsorbent material, add to the expense and complexity of preparing the superabsorbent material, and may negatively affect other liquid handling properties of the superabsorbent material.
Commercially available superabsorbents are generally in a substantially neutralized or salt form. This is because, in general, in order to have a relatively high capacity for liquid absorption, a water-swellable, water-insoluble polymer must be a polyelectrolyte. However, such superabsorbents absorb liquid relatively quickly, leading to the above discussed problems. It is known that when an acidic or basic water-swellable, water-insoluble polymer, substantially in its free acid or free base form, respectively, is mixed with a basic second material or an acidic second material, respectively, the resulting absorbent composition will exhibit both a relatively high capacity for liquid absorption as well as a relatively slow liquid absorbing rate. This is believed to be because, as the mixture is placed in an aqueous solution, the acidic or basic water-swellable, water-insoluble polymer, substantially in its free acid or free base form, respectively, reacts with the basic second material or the acidic second material, respectively, and the chemical equilibrium is in favor of converting the acidic or basic water-swellable, water-insoluble polymer from its free acid or free base form, respectively, to its respective salt form. As such, the mixture comprising the converted water-swellable, water-insoluble polymer will now exhibit a relatively high capacity for liquid absorption. However, because the conversion of the water-swellable, water-insoluble polymer, from its free acid or free base form, respectively, to its respective salt form is a relatively slow process of ionization and ion diffusion into the polymer, the water-swellable, water-insoluble polymer will also exhibit a relatively slow liquid absorbing rate. In addition, the conversion of the water-swellable, water-insoluble polymer from its free acid or free base form to its respective salt form in an electrolyte-containing solution, such as an aqueous sodium chloride solution, has a substantial desalting effect on the electrolyte-containing solution, thereby improving the liquid-absorbing performance of the mixture by alleviating the salt-poisoning effect.
WO 98/24832 discloses an absorbent composition that includes a polymeric absorbent material and a second material. The two components are mixed and used in an absorbent article, desirably in conjunction with a fibrous matrix. The second material can be provided in the form of particles, flakes, fibers, films, and nonwoven structures or the two components can be provided as a bi-component fiber. This composition has disadvantages in terms of handling properties and structural integrity.
A single material or polymer comprising both acidic and basic functional groups within its molecular structure will not exhibit the desired absorbent properties described above. This is believed to be because such acidic and basic functional groups within a single molecular structure will typically react with each other and might result in an over-crosslinked polymer structure. As such, it generally is not possible to prepare an absorbent composition by preparing a copolymer from acidic and basic monomers or by preparing a molecular level dispersion, such as in an aqueous solution, of water-soluble acidic and basic materials since during such copolymerization or molecular level dispersion the acidic and basic materials will typically react with each other and crosslink.
Accordingly, it is an object of the present invention to provide a fibrous matrix that releases an activating agent upon contact with an insult liquid, wherein the activating agent causes an absorbent material to form a superabsorbent polymer.
It is also an object of the present invention to provide an absorbent structure that will form a superabsorbent polymer in situ upon contact with a liquid to be absorbed.
The present invention is directed to absorbent structures that form superabsorbent polymers in situ. The structures include an absorbent material and a fibrous material containing an activating agent. The fibrous material releases the activating agent, which causes the polymer to become a superabsorbent polymer (SAP). The absorbent component is desirably a water-swellable, water-insoluble polymer.
The fibrous material releases the activating agent upon stimulation with an activator, which desirably is the liquid that is to be absorbed by the superabsorbent. This liquid will generally be an aqueous liquid such as urine or another bodily fluid. The fibrous material desirably releases the activating agent over a period of time, desirably at a controlled rate.
The absorbency of the SAP is greater than the absorbency of the fibrous material or the absorbent material separately. As the activating agent is released from the fibrous material at a controlled rate, the absorbent polymer gains the functional characteristics of a hydrophilic SAP over a period of time. In one embodiment, the SAP is a slow SAP.
In one aspect, therefore, the present invention is absorbent structures that form a superabsorbent composition in situ that absorbs liquid and is able to absorb a relatively large quantity of the liquid.
In one embodiment, the polymer is polyacrylic acid gel, and the fibrous material carries a basic activating agent, such as sodium bicarbonate, sodium carbonate or a mixture thereof.
In another aspect, the invention is a fibrous material that contains an activating agent that it can release over a period of time, desirably at a controlled rate. The fibrous material can be combined with an absorbent, wherein release of the activating agent causes the absorbent to act as an SAP. In one embodiment, the fibrous material is a cellulosic fiber having sodium bicarbonate, sodium carbonate, or a mixture thereof incorporated therein.
In another aspect, the invention is methods for forming a superabsorbent polymer in situ.
In one embodiment, the methods include providing a fibrous material containing an activating agent that the fibrous material releases upon contact with the activating liquid and providing a water-swellable, water-insoluble polymer that will form a superabsorbent upon contact with the activating agent. When the activating liquid is brought into contact with the fibrous material, the activating agent is released, whereupon the activating agent causes the water-swellable, water-insoluble polymer to form a superabsorbent.
In another aspect, the invention is a disposable absorbent product that includes an absorbent structure of the present invention. In one embodiment, a disposable absorbent product includes a liquid-permeable topsheet, a backsheet attached to the topsheet, and an absorbent structure of the present invention positioned between the topsheet and the backsheet.
An absorbent structure is disclosed that forms a superabsorbent material that exhibits a relatively high total liquid absorption capacity. In one embodiment, the SAP absorbs liquid at a relatively slow liquid absorbing rate. The absorbent structure includes at least two components: an absorbent material and a fibrous material that releases an activating agent that causes the absorbent material to become a superabsorbent polymer. One of the components is acidic and one is basic, and the absorbent structure forms an absorbent composition that exhibits significant and unexpected improvements in its absorbent properties, including improved total liquid absorption as well as, optionally, a slower liquid absorbing rate, as compared to the properties exhibited by either of the components alone.
As used herein, an xe2x80x9cacidicxe2x80x9d material refers to a material that may act as an electron acceptor.
As used herein, a xe2x80x9cbasicxe2x80x9d material refers to a material that may act as an electron donor.
As used herein, the term xe2x80x9ccellulosicxe2x80x9d or xe2x80x9ccellulosexe2x80x9d is meant to include any material having cellulose as a major constituent, and specifically, comprising at least 50 percent by weight cellulose or a cellulose derivative. Thus, the term includes cotton, typical wood pulps, cellulose acetate, cellulose triacetate, rayon, thermomechanical wood pulp, chemical wood pulp, debonded chemical wood pulp, milkweed, and the like.
As used herein, the term xe2x80x9cwater-swellable, water-insolublexe2x80x9d refers to a material that, when exposed to an excess of water, swells to its equilibrium volume but does not dissolve into the solution. As such, a water-swellable, water-insoluble material generally retains its original identity or physical structure, but in a highly expanded state, during the absorption of the water and, thus, must have sufficient physical integrity to resist flow and fusion with neighboring particles. As used herein, a material will be considered to be water soluble when it substantially dissolves in excess water to form a solution, thereby losing its initial, typically particulate, form and becoming essentially molecularly dispersed throughout the water solution. As a general rule, a water-soluble material will be free from a substantial degree of crosslinking, as crosslinking tends to render a material water-insoluble.
The term xe2x80x9csuperabsorbentxe2x80x9d as used herein refers to a material capable of absorbing large quantities of liquids such as water and body exudates in relation to its weight and forming hydrogels thereby. Superabsorbents are often capable of retaining absorbed fluids under moderate pressure. Hydrogels are also referred to as hydrocolloids and gels.
The term xe2x80x9cslow, superabsorbentxe2x80x9d as used herein refers to a superabsorbent having an absorption time index of at least 5 minutes and desirably more than 10 minutes. The absorption time index is defined as the time for a SAP to swell to 60% of its free swelling capacity.
xe2x80x9cPapermaking fibers,xe2x80x9d as used herein, include all known cellulosic fibers or fiber mixes comprising cellulosic fibers. Fibers suitable for making the webs of this invention comprise any natural or synthetic cellulosic fibers including, but not limited to: nonwoody fibers, such as cotton fibers and cotton derivatives, abaca, kenaf, sabai grass, flax, esparto grass, straw, jute hemp, bagasse, milkweed floss fibers, and pineapple leaf fibers; and woody fibers such as those obtained from deciduous and coniferous trees, including softwood fibers, such as northern and southern softwood kraft fibers; hardwood fibers, such as eucalyptus, maple, birch, aspen, or the like. Wood fibers may be prepared in high-yield or low-yield forms and may be pulped in any known method, include kraft, sulfite, groundwood, thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP) and bleached chemithermomechanical pulp (BCTMP). High brightness pulps, including chemically bleached pulps, are especially desired for tissue making, but unbleached or semi-bleached pulps may also be used. Recycled fibers are included within the scope of the present invention. Any known pulping and bleaching methods may be used.
Synthetic cellulose fiber types include rayon in all its varieties and other fibers derived from viscose or chemically modified cellulose. Chemically treated natural cellulosic fibers may be used such as mercerized pulps, chemically stiffened or crosslinked fibers, sulfonated fibers, and the like. Suitable papermaking fibers may also include recycled fibers, virgin fibers, or mixes thereof.
As used herein, xe2x80x9cmercerizationxe2x80x9d is meant to refer to a process wherein cellulosic fibers are treated under suitable conditions to convert the cellulose from its native form, sometimes referred to as cellulose I, into another crystalline form, sometimes referred to as cellulose II. Because the chemically treated or mercerized cellulose is less crystalline and more amorphous, the chemically treated or mercerized cellulose is generally more accessible for further treatment with additional reagents. Known test methods exist for quantifying the chemical treatment, such as the degree of mercerization that occurs, of cellulosic fibers.
The absorbent compositions of the present invention generally include at least two components. The first component is a water-swellable, water-insoluble polymer. As used in the absorbent structure of the present invention, the water-swellable, water-insoluble polymer to a large extent needs to provide the absorbent composition with its liquid-absorbing capacity. As such, the water-swellable, water-insoluble polymer needs to be able to be effective to provide a desired amount of liquid-absorbing capacity to the absorbent composition. The second component is a fibrous material having incorporated therein an activating agent that it releases over a period of time. The activating agent acts on the water-swellable, water-insoluble polymer to form a superabsorbent polymer (SAP). The activating agent is desirably a neutralizing agent that neutralizes the acidic or basic absorbent material, causing it to become a SAP.
By xe2x80x9creleases the activating agentxe2x80x9d it is meant herein that the fibrous material releases all or a portion of the compound that is incorporated into or onto the fibrous material. Only a portion of the incorporated compound may be released and all or a portion of the released compound may act upon the water-swellable water-insoluble polymer to cause it to become a super absorbent polymer.
The SAP formed by the absorbent structure suitably has the ability to absorb a liquid, herein referred to as Gel Capacity. The method by which the Gel Capacity is determined is set forth below. As a general rule, it is desired that the SAP of the present invention has a Gel Capacity value, for a load of about 0.01 psi, of at least about 10, more desirably of at least about 20, even more desirably of at least about 25, most desirably of at least about 35, and up to about 200 grams per gram. The gel capacity desirably ranges from about 30 to about 45.
A. The Polymer
One property of the water-swellable, water-insoluble polymer that is relevant to its effectiveness in providing a desired amount of liquid-absorbing capacity to the absorbent composition is its molecular weight. In general, a water-swellable, water-insoluble polymer with a higher molecular weight will exhibit a higher liquid-absorbing capacity as compared to a water-swellable, water-insoluble polymer with a lower molecular weight.
The water-swellable, water-insoluble polymer useful in the absorbent composition may generally have a wide range of molecular weights. A water-swellable, water-insoluble polymer having a relatively high molecular weight is often beneficial for use in the present invention. Nonetheless, polymers having a wide range of molecular weights are generally suitable for use in the present invention. Water-swellable, water-insoluble polymers suitable for use in the present invention will beneficially have a weight average molecular weight greater than about 100,000, more beneficially greater than about 200,000, suitably greater than about 500,000, more suitably greater than about 1,000,000, and up to about 10,000,000. Methods for determining the molecular weight of a polymer are generally well known in the art.
It is sometimes more convenient to express the molecular weight of a polymer in terms of its viscosity in a 1.0 weight percent aqueous solution at 25 xc2x0 C. Polymers suitable for use in the present invention will suitably have a viscosity in a 1.0 weight percent aqueous solution at 25 xc2x0 C. of from about 100 centipoise (100 mPaxc2x7s) to about 80,000 centipoise (80,000 mPaxc2x7s), more suitably from about 500 centipoise (500 mPaxc2x7s) to about 80,000 centipoise (80,000 mPaxc2x7s), and most suitably from about 1,000 centipoise (1,000 mpaxc2x7s) to about 80,000 centipoise (80,000 mPaxc2x7s).
The water-swellable, water-insoluble polymer useful in the absorbent composition will generally be crosslinked. The amount of crosslinking should generally be above a minimum amount sufficient to make the polymer water-insoluble but also below some maximum amount so as to allow the polymer to be sufficiently water swellable so that the water-swellable, water-insoluble polymer absorbs a desired amount of liquid.
Crosslinking of the polymer may generally be achieved by either of two different types of crosslinking agents. The first type of crosslinking agent is a polymerizable crosslinking agent. Suitable polymerizable crosslinking agents are generally reactive to the monomer or monomers used to prepare the polymer and, thus, generally comprise at least two functional groups that are capable of reacting with the monomers. Examples of suitable polymerizable crosslinking agents include ethylenically unsaturated monomers, such as N,Nxe2x80x2-methylene bis-acrylamide, for free radical polymerization, and polyamines or polyols for condensation polymerization.
The second type of crosslinking agent is a latent crosslinking agent. Latent crosslinking agents generally do not take part in the overall polymerization process but, instead, are reactive to the polymer at a later point in time when a proper crosslinking condition is provided. Suitable post treatment conditions include using heat treatment, such as a temperature above about 60xc2x0 C., exposure to ultraviolet light, exposure to microwaves, steam or high humidity treatment, high pressure treatment, or treatment with an organic solvent.
Latent crosslinking agents suitable for use in the present invention are generally water soluble. A suitable latent crosslinking agent is an organic compound having at least two functional groups or functionalities capable of reacting with any carboxyl, carboxylic, amino, or hydroxyl groups on the polymer. Examples of suitable latent crosslinking agents include, but are not limited to, diamines, polyamines, diols, polyols, polycarboxylic acids, and polyoxides. Another suitable latent crosslinking agent comprises a metal ion with more than two positive charges, such as Al3+, Fe3+, Ce3+; Ce4+, Ti4+, Zr4+, and Cr3+.
When the polymer is a cationic polymer, a suitable crosslinking agent is a polyanionic material such as sodium polyacrylate, carboxymethyl cellulose, or polyphosphate.
The water-swellable, water-insoluble polymer useful in the absorbent composition will either be acidic or basic in nature. In general, weakly acidic water-swellable, water-insoluble polymers will provide slower superabsorbents whereas strongly acidic polymers will provide superabsorbents having faster action. Accordingly, the polymer can be chosen in part based upon the desired speed of absorbency of the formed superabsorbent polymer.
Suitable acidic water-swellable, water-insoluble polymers will include functional groups that are capable of acting as acids. Such functional groups include, but are not limited to, carboxyl groups, sulfate groups, sulfite groups, and phosphate groups. Suitably, the functional groups are carboxyl groups. Generally, the functional groups are attached to a crosslinked base polymer. Suitable base polymers include polyacrylamides, polyvinyl alcohols, ethylene maleic anhydride copolymer, polyvinylethers, polyacrylic acids, polyvinylpyrrolidones, polyvinylmorpholines, and copolymers thereof. Natural based polysaccharide polymers may also be used and include carboxymethyl celluloses, carboxymethyl starches, hydroxypropyl celluloses, algins, alginates, carrageenans, acrylic grafted starches, acrylic grafted celluloses, and copolymers thereof. Synthetic polypeptides can also be used such as polyaspartic acid and polyglutamic acid. Copolymers and mixtures of the above can also be used.
The acidic, water-swellable, water-insoluble polymer generally needs to be in its free acid form. In general, it is desired that the acidic, water-swellable, water-insoluble polymer beneficially have at least about 50 molar percent, more beneficially at least about 70 molar percent, suitably at least about 80 molar percent, more suitably at least about 90 molar percent, and most suitably substantially about 100 molar percent of its acidic functional groups in free acid form. Alternatively, the acidic, water-swellable, water-insoluble polymer should not be substantially neutralized when used in the absorbent composition of the present invention. In general, it is desired that the acidic, water-swellable, water-insoluble polymer have a degree of neutralization of its acidic functional groups that is beneficially less than about 50 molar percent, more beneficially less than about 30 molar percent, suitably less than about 20 molar percent, more suitably less than about 10 molar percent, and most suitably substantially about 0 molar percent.
Desired acidic water-swellable, water-insoluble polymers useful in the invention are polyacrylic acid, ethylene maleic anhydride copolymer, acrylic acid grafted starch, acrylic acid grafted cellulose, carboxymethyl cellulose, and polyacrylamide methylpropane sulfonic acid. Most desired acidic water-swellable, water-insoluble polymers include polyacrylic acid, polyacrylamide methylpropane sulfonic acid, and acrylic acid grafted starch.
In one embodiment of the invention, where the desire is to make slow superabsorbents, the acidic, water-swellable, water-insoluble polymers used in the absorbent composition will be weakly acidic in nature. Such polymers will beneficially have a pKa between about 2 to about 12, more beneficially between about 2 to about 10, and most desirably between about 3 to about 6. It may sometimes be more convenient to measure the pH of the monomer or monomers used to prepare a polymer. Although the pH of the monomer or monomers and the polymer prepared from such monomers may not be identical, such pH values should be substantially similar. As such, acidic, water-swellable, water-insoluble polymers useful in the absorbent composition will be prepared from monomers that beneficially have a pKa between about 2 to about 12, more beneficially between about 2 to about 10, and most desirably between about 3 to about 6. If a polymer is prepared from two or more monomers, each of the monomers used should beneficially have a pKa between about 2 to about 12, more beneficially between about 2 to about 10, and most desirably between about 3 to about 6, although insubstantial amounts of monomers having a pKa less than about 2 or greater than about 12 may be used as long as such monomers do not negatively affect the desired properties of the water-swellable, water-insoluble polymer useful herein.
The pKa of an acid represents the extent of dissociation of or, in other words, the strength of the acid and is intended herein to be measured at the conditions, such as at a specific temperature, under which the water-swellable, water-insoluble polymer is being used. Suitably, the pKa is measured at about 25 xc2x0 C. In general, the weaker the acid, the higher the pKa value will be. The pKa values for many acids at various temperatures are well known and may be found in any of many available references, such as in the CRC Handbook of Chemistry and Physics, 75th Edition, edited by David R. Lide, CRC Press (1994).
Suitable basic, water-swellable, water-insoluble polymers will include functional groups that are capable of acting as bases. Such functional groups include, but are not limited to, primary, secondary, or tertiary amino groups, imino groups, imido groups, and amido groups. Suitably the functional groups are amino groups. Generally, the functional groups are attached to a crosslinked base polymer. Suitable base polymers include polyamines, polyethyleneimines, polyacrylamides, polydiallyl dimethyl ammonium hydroxide, and polyquaternary ammoniums. Natural based polysaccharide polymers may also be used and include chitin and chitosan. Synthetic polypeptides can also be used such as polyasparagines, polyglutamines, polylysines, and polyarginines. Mixtures and copolymers of the foregoing can also be used.
The basic, water-swellable, water-insoluble polymer generally needs to be in its free base form. In general, it is desired that the basic, water-swellable, water-insoluble polymer beneficially have at least about 50 molar percent, more beneficially at least about 70 molar percent, suitably at least about 80 molar percent, more suitably at least about 90 molar percent, and most suitably substantially about 100 molar percent of its basic, functional groups in free base form. Alternatively, the basic, water-swellable, water-insoluble polymer should not be substantially neutralized when used in the absorbent composition of the present invention. In general, it is desired that the basic, water-swellable, water-insoluble polymer have a degree of neutralization of its basic functional groups that is beneficially less than about 50 molar percent, more beneficially less than about 30 molar percent, suitably less than about 20 molar percent, more suitably less than about 10 molar percent, and most suitably substantially about 0 molar percent.
Desired basic water-swellable, water-insoluble polymers useful in the invention include chitosan, quaternary ammoniums, polyvinylamine, polyethylene imine, and polydialkylaminoalkyl methacrylamide. Most desired basic water-swellable, water-insoluble polymers useful in the invention include quaternary ammoniums, polyvinylamine, and polyethylene imine.
In one embodiment of the invention, where the desire is to make slow superabsorbents, the basic, water-swellable, water-insoluble polymer will be weakly basic in nature. Such polymers will beneficially have a pKb between about 2 to about 12, more beneficially between about 2 to about 10, and suitably between about 3 to about 6. It may sometimes be more convenient to measure the pH of the monomer or monomers used to prepare a polymer. Although the pH of the monomer or monomers and the polymer prepared from such monomer or monomers may not be identical, such pH values should be substantially similar. As such, basic, water-swellable, water-insoluble polymers useful in the absorbent composition will be prepared from monomers that beneficially have a pKb between about 2 to about 12, more beneficially between about 2 to about 10, and suitably between about 3 to about 6. If a polymer is prepared from two or more monomers, each of the monomers used should beneficially have a pKb between about 2 to about 12, more beneficially between about 2 to about 10, and suitably between about 3 to about 6, although insubstantial amounts of monomers having a pKb less than about 2 or greater than about 12 may be used as long as such monomers do not negatively affect the desired properties of the water-swellable, water-insoluble polymer useful herein.
The pKb of a base represents the extent of dissociation of or, in other words, the strength of the base and is intended herein to be measured at the conditions, such as at a specific temperature, under which the water-swellable, water-insoluble polymer is being used. Suitably, the pKb is measured at about 25 xc2x0 C. In general, the weaker the base, the higher the pKb value will be. The pKb values for bases at various temperatures are well known and may be found in any of many available references, such as in the CRC Handbook of Chemistry and Physics, 75th Edition, edited by David R. Lide, CRC Press (1994).
The acidic or basic water-swellable, water-insoluble polymer may generally be used in the absorbent composition in a variety of forms. Examples of forms that the acidic or basic water-swellable, water-insoluble polymer may take include particles, flakes, fibers, films, and nonwoven structures. When the absorbent structure is used in absorbent disposable products, it is generally desired that the acidic or basic water-swellable, water-insoluble polymer be in the form of discrete particles, fibers, or flakes. When in the form of a particle, it is generally desired that the particle have a maximum cross-sectional dimension beneficially within the range from about 50 micrometers to about 2,000 micrometers, suitably within the range from about 100 micrometers to about 1,000 micrometers, and more suitably within the range from about 300 micrometers to about 600 micrometers.
B. The Fibrous Material
The fibrous material is desirably a fibrous matrix having a form such as a fibrous network, which is, generally, a random plurality of fibers that can, optionally, be joined together with a binder. The fibrous material can alternatively have the form of a batt of comminuted wood pulp fluff, a tissue layer, a hydroentangled pulp sheet, a woven sheet, a nonwoven sheet, a tow, or a mechanically softened pulp sheet. In desired embodiments, the fibrous material is similar to the fibrous material that otherwise would be used in the absorbent article.
Any papermaking fibers, as previously defined, or mixtures thereof may be used that can retain the activating agent and release it, desirably in a controlled fashion over a period of time. Because of commercial availability, softwood and hardwood fibers are especially desired. In one embodiment, the fibers may be predominantly hardwood, such as at least 50% hardwood or about 60% hardwood or greater or about 80% hardwood or greater or substantially 100% hardwood. Higher hardwood contents are desired for high opacity and softness, whereas higher softwood content is desirable for strength. In another embodiment, the fibers may be predominantly softwood, such as at least 50% softwood or about 60% softwood or greater or about 80% softwood or greater or substantially 100% softwood.
The fibrous material may be formed from a single layer or multiple layers. In the case of multiple layers, the layers are generally positioned in a juxtaposed or surface-to-surface relationship and all or a portion of the layers may be bound to adjacent layers. The fibrous material may also be formed from a plurality of separate fibrous webs wherein the separate fibrous webs may be formed from single or multiple layers.
To achieve good softness and opacity, it is desirable that the fibrous component comprise substantial amounts of hardwood. For good strength, substantial amounts of softwood are desired. Both strength and softness are often achieved through layered tissues, such as those produced from stratified headboxes wherein at least one layer delivered by the headbox comprises softwood fibers while another layer comprises hardwood or other fiber types. Layered tissue structures produced by any means known in the art are within the scope of the present invention, including those disclosed by Edwards et al. in U.S. Pat. No. 5,494,554.
The fibrous material contains the activating agent and can release the activating agent, desirably in a controlled manner. By controlled manner is meant at an approximately constant rate over a period of time. The fibrous material desirably releases the activating agent upon contact with an activator, which is usually an insult liquid which is to be absorbed by the superabsorbent. The activating agent can be retained on the fibrous material by a number of means. For example, the activating agent can be retained by water labile bonds, such as ionic bonds or water hydrolyzable covalent bonds. The activating agent can also be retained by physical means, such as encapsulation or physical entrapment.
The rate of release of the activating agent from the fibrous material is controlled by several factors. One factor is the concentration of the activating agent and the nature of the activating agent such as its molecular weight, charge, and solubility. Another factor is the type and strength of the bond or interaction between the agent and the fiber. For example, an activating agent that is carried within the structure of the fibrous material will likely be released more slowly than an activating agent carried on the matrix surface.
C. The Activating Agent
If the water-swellable, water-insoluble polymer is acidic, the activating agent will be basic. Alternatively, if the water-swellable, water-insoluble polymer is basic, the activating agent will be acidic. Examples of basic activating agents are carbonates and bicarbonates such as sodium bicarbonate and sodium carbonate, polyamines, polyimines, polyamides, polyquaternary ammoniums, chitins, chitosans, polyasparagines, polylysines, polyarginines, aliphatic amines, aromatic amines, imines, amides, metallic oxides, hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, salts, ammonia, and mixtures and copolymers thereof. Desired basic activating agents are sodium bicarbonate, sodium carbonate, potassium carbonate, and potassium bicarbonate.
Examples of acidic activating agents are polyacrylic acid, polymaleic acid, carboxymethyl cellulose, alginic acid, polyaspartic acid, polyglutamic acid, citric acid, glutamic acid, aspartic acid, inorganic acid, salts, isosaccharinic acid, tartaric acid, oxalic acid, malonic acid, glucuronic acid and mixtures and copolymers thereof. Desired acidic activating agents are citric acid, glutamic acid, aspartic acid, ammonium chloride, and calcium chloride.
The activating agent is included in the fibrous material at about 5 to 80 weight percent, desirably at about 10 to 50 weight percent, and most desirably at about 20 to 40 weight percent.
The activating agent and the polymer should be present in the absorbent structure in an acid to base ratio from about 1:5 to 5:1, desirably from about 1:2 to 2:1, and most desirably at about a 1:1 acid to base ratio.
When the activating agent is basic, the pH of the fibrous material ranges from about 7.5 to 14, and is desirably about 7.5 to 9.0. When the activating agent is acidic, the pH of the fibrous material ranges from about 0 to 6.5, and is desirably about 4 to 6.5.
D. Buffering Agent
Optionally, a buffering agent can be included to maintain the pH of the surface of the products. Buffering agents that can be used include aspartic acid, ascorbic acid, chloroacetic acid, xcex2-chlorobutyric acid, cis-cinnamic acid, citric acid, fumaric acid, glutaramic acid, glutaric acid, itaconic acid, lactic acid, malic acid, malonic acid, o-phthalic acid, succinic acid, xcex1-tartaric acid, and phosphoric acid, xcex1-alanine, allantoin, cysteine, cystine, dimethylglycine, histidine, glycine, chitosan, N-(2-acetamido)-2-iminodiacetic acid, tris(hydroxymethyl) aminomethane, theobromine, and tyrosine.
While the principal components of the absorbent composition of the present invention have been described in the foregoing, such absorbent composition is not limited thereto and can include other components not adversely effecting the absorbent composition having the desired absorbent properties. Exemplary materials which could be used as additional components include, without limitation, pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters, solid solvents, particulates, and materials added to enhance processability of the absorbent composition.
The water-swellable water-insoluble polymers are commercially available or can be made by methods well known in the art. The fibrous material can also be made by methods known to those skilled in the art. The activating agent may be incorporated into or onto the fibrous material during or after the formation of the general form of the fibrous material. A fibrous material useful in the present invention may be formed by an air-laying process or a wet-laid process, or by essentially any other process known to those skilled in the art for forming a fibrous material.
In general, the activating agent is incorporated into or onto the fibrous material by a process which involves reversibly bonding the agent to the material, reversibly encapsulating the agent into the material, or reversibly physically entrapping the agent in the fibrous material. Accordingly, the interaction between the activating agent and the fibrous material can be an ionic interaction, a labile covalent interaction, or a physical interaction. Combinations of these types of processes and interactions can also be used.
In one embodiment, the process involves dispersing the fibrous material into a liquid phase that includes at least one component that can loosen the fibrous material""s supermolecular structure, such as sodium hydroxide or potassium hydroxide. This component may be retained by the fibrous material or may be washed away. The activating agent or a compound that will form the agent is then added to the dispersion. The agent or compound is desirably soluble in the liquid phase. The liquid phase is then removed so that the cellulosic fiber encapsulates, or retains, the activating agent inside its supermolecular structure.
In one embodiment, the activated agent containing fibrous material is prepared using mercerized cellulosic fibrous material. Any of a number of methods of mercerization can be used, a desired method is that disclosed in U.S. Pat. No. 5,858,021 to Sun et al. That method involves preparation of a first mixture of cellulosic fibers and water, desirably at a consistency of between about 27 to 100 weight percent, most desirably between about 40 and 80 weight percent. An alkali metal hydroxide is added to the first mixture to make a second mixture. The alkali metal hydroxide is desirably sodium hydroxide, potassium hydroxide, lithium hydroxide, or mixtures thereof. Tha alkali metal hydroxide is desirably added at an amount of between about 5 to 55 weight peercent of the second mixture, more desirably between about 13 to 50 weight percent, based upon the total weight of alkali metal hydroxide and water in the mixture. The second mixture is homogenized and, desirably, heated to about 60xc2x0 C. to 80xc2x0 C. to effectively mercerize the fibers.
The mercerized fibrous material is then quickly rinsed, but not washed as is standard after mercerization processes. The fiber is exposed to carbon dioxide and a mixture of sodium bicarbonate and sodium carbonate is formed and embedded inside the fiber wall, the lumen, and a limited amount will be retained on the fiber surface. This method provides an advantage in terms of preparation of the fiber and the absorbent material in that the fiber does not have to be washed after NaOH treatment, as is usually done in mercerization processes.
Absorbent structures can be made in the same way as absorbent structures without the activating agent treated fibrous material.
The absorbent structures of the present invention are suitable for use in disposable absorbent products such as personal care products, such as diapers, training pants, baby wipes, feminine care products, adult incontinent products; and medical products, such as wound dressings, surgical capes, and drapes. When the absorbent structure of the present invention is intended for use in disposable absorbent products, it is typically desired that the resultant superabsorbent composition have a generally neutral or slightly acidic character. For such a particular use, it is generally desired that the molar ratio of acidic or basic water-swellable, water-insoluble polymer to basic or acidic activating agent be about 1:1.
In one embodiment of the present invention, a disposable absorbent product is provided, which includes a liquid-permeable topsheet, a backsheet attached to the topsheet, and an absorbent structure of the invention positioned between the topsheet and the backsheet.
Disposable absorbent products, according to all aspects of the present invention, are generally subjected during use to multiple insults of a body liquid. Accordingly, the disposable absorbent products are desirably capable of absorbing multiple insults of body liquids in quantities to which the absorbent products and structures will be exposed during use. The insults are generally separated from one another by a period of time.
Those skilled in the art will recognize materials suitable for use as the topsheet and backsheet. Exemplary of materials suitable for use as the topsheet are liquid-permeable materials, such as spunbonded polypropylene or polyethylene having a basis weight of from about 15 to about 25 grams per square meter. Exemplary of materials suitable for use as the backsheet are liquid-impervious materials, such as polyolefin films, as well as vapor-pervious materials, such as microporous polyolefin films.
The absorbent structure of the present invention should be present in an amount effective to form superabsorbent composition effective to result in the absorption of a desired amount of liquid.
The invention is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the scope thereof. On the contrary, it is to be clearly understood that resort may be had to various other embodiments, modifications, and equivalents thereof, which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the present invention.